Fine powder materials synthesis is finding particular application in the fields of powder metallurgy, semiconductors, magnetics and ceramics. In each of these fields, the synthesis of high-purity, nanometer-sized particles or "nano-particles" is considered highly desirable. Primary nanoparticles in the 1-100 nm size range permit the creation of materials with carefully controlled properties. In view of the desirability of the particles, as described, several methods for synthesizing sub-micron particles have been developed.
The generation of fine, pure, uniform, spherical, particles is of intense interest because of their recently recognized properties as suitable starting materials for producing high performance, dense ceramic articles. Densified bodies produced from such powders are predicted to be very strong and to have significantly enhanced property reproducibility. Silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4) are two ceramic materials currently considered highly suitable for use in advanced military and civilian engines.
The direct synthesis of such ceramic powders from gas phase reactants has been achieved using lasers, RF plasma heating systems and heated flow tubes. The first two methods have the advantage over other methods, such as solid phase synthesis and chemical vapor deposition, of avoiding contact of the reactants or products with hot walls (a source of contamination). The latter two methods suffer from non-uniformities in the size of the reaction zone resulting in the production of undesirable wide particle size distribution, agglomeration, etc. The first system is difficult to scale from the laboratory to a production facility.
Various physical, chemical and mechanical methods have been devised for the synthesis of nanostructured powders (n-powders). These have been described in detail in the scientific literature (see "NanoStructured Materials," Vols. I, II and III, 1992-4). Of particular relevance to this invention is the prior art on the synthesis of n-powders by (1) thermal decomposition of metallo-organic precursors using a focused laser beam, combustion flame or plasma torch as heat source, and (2) evaporation and condensation of volatile species in a reduced-pressure environment.
Nanosized particles have distinctly different properties compared to bulk materials because the number of atoms on the particle surface is comparable to that inside the particle (Andres, R. P., R. S. Averback, W. L. Brown, L. E. Brus, W. A. Goddard III, A. Kaldor, S. G. Louie, M. Moscovits, P. S. Peercy, S. J. Riely, R. W. Siegel, F. Spaepen, and Y. Wang, "Research Opportunities on Cluster and Cluster-Assembled Materials--A Department of Energy, Council on Materials Science Panel Report, J. Meter. Res., 4, 704 (1989)). As a result, these particles are characterized by lower melting point, better light absorption and structural properties. Nanosized particles are also used to form catalysts with high specific surface area and large density of active sites. Though a number of processes have been developed for synthesis of nanoparticles, their production cost remains high, limiting, thus, the development of their applications. Flame reactors, on the other hand, are routinely used in industrial synthesis of submicron powders with relatively narrow size distribution and high purity (Ulrich, G. D., "Flame Synthesis of Fine Particles", C&EN, 62(8), 22 (1984)).
Charging particles during their formation can have a profound effect on the product particle characteristics: primary particle size, crystallinity, degree of aggregation and agglomerate size. Hardesty and Weinberg ("Electrical Control of Particulate Pollutants from Flames", Fourteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1365 (1973)) showed that the silica primary particle size can be reduced by a factor of three when an electric field is applied across a counterflow CH.sub.4 /air diffusion flame. They attributed it to the rapid deposition of particles on the electrodes, thus decreasing the particle residence time in the high temperature region of the flame. Likewise, Katz and Hung ("Initial Studies of Electric Field Effects on Ceramic Powder Formation in Flames", Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh 1733 (1990)) showed that the size of TiO.sub.2, SiO.sub.2 and GeO.sub.2 particles made in a similar reactor were greatly influenced by the presence of electric fields. Xiong et al. (1992) showed theoretically that charging titania particles unipolarly during their synthesis can reduce the particle size and narrow the particle size distribution.
Titania is used as a pigmentary material (Mezey, E. J., "Pigments and Reinforcing Agents" in VAPOR DEPOSITION, C. F. Powell, J. H. Oxley and J. M. Blocher, Jr., (Eds.), John Wiley & Sons, New York, 423 (1966)), photocatalyst (Ollis, D. F., Pelizzetti, E., and N. Serpone, "Photocatalytic Destruction of Water Contaminants", Environ. Sci. Tech., 25, 1523 (1991)), and as a catalyst support (Bankmann et al., 1992). Fumed silica particles are widely used for optical fibers, catalyst supports and as a filler and dispersing agent (Bautista, J. R., and R. M. Atkins, "The Formation and Deposition of SiO.sub.2 Aerosols in Optical Fiber Manufacturing Torches", J. Aerosol Sci., 22, 667 (1991)). Nanosized tin oxide powders are used as a semiconductor and gas sensor (Kim, E. U-K., and I. Yasui, "Synthesis of Hydrous SnO.sub.2 and SnO.sub.2 -Coated TiO.sub.2 Powders by the Homogeneous Precipitation Method and their Characterization", J. Mater. Sci., 23, 637 (1988)). The objectives of the present invention are to provide methods using plate electrodes across the premixed flame for synthesis of nanophase materials with closely controlled characteristics.
Flame aerosol technology refers to the formation of fine particles from gases in flames. This technology has been practices since prehistoric times as depicted with paintings in cave walls and Chinese ink artwork. Today flame technology is employed routinely in large scale manufacture of carbon blacks and ceramic commodities such as fumed silica and pigmentary titania and, to a lesser extent, for specialty chemicals such as zinc oxide and alumina powders. These powders find most of their applications as pigments and reinforcing agents and, relatively recently, in manufacture of optical waveguides. Today the production volume of this industry is in the order of millions metric tons per year worldwide. Though this is an established industrial process bringing sizable profits to the corresponding corporations, its fundamentals are not yet well understood. This lack of understanding makes truly difficult the process development and scale-up for manufacture of titania, silica and other ceramic particles of closely controlled size including nanoparticles.
According to flame technology, vapor of the precursor compound reacts at high temperature with oxygen or any other desirable oxidant or gas resulting in the product ceramic powder in the form of a cluster of cemented primary particles. The size of primary particles ranges from a few to several hundred nanometers in diameter depending on material and process conditions. In most industrial processes, especially in the oxidation of SiCl.sub.4 or TiCl.sub.4, these reactions are exothermic so little additional fuel is needed to initiate or sustain the process and the ensuring flame. These powders are collected by conventional means (cyclones and baghouse filters) downstream of the flame reactor as the gas cools down.
The control of particle characteristics during flame synthesis is crucial because the properties of materials made from these particles depend on size and size distribution, morphology, extent of agglomeration and chemical and phase composition. For example, in the manufacture of titania pigments, the goal is to produce a nearly monodisperse rutile phase particle about 250 nm in diameter resulting in maximum hiding powder per unit mass. In contrast, in manufacture of powders for structural ceramics, particle size is not so important. There, agglomerates should be avoided since they result in pores and flaws during sintering reducing, thus, the strength of the final part or specimen made with these particles.
Today oxides like SiO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3, GeO.sub.2, V.sub.2 O.sub.5, and most other oxides of metal elements in the periodic table and their composites have been produced in powder form in hydrocarbon flames on a laboratory scale. These powders are made in premixed and coflow or counterflow diffusion flame reactors. More recently, flame processes have been developed for gas phase synthesis of non-oxide powders such as silicon nitride (H. F. Calcote, W. Felder, D. G. Keil, D. B. Olson, Twenty-third Symposium (Int.) on Combustion, the Combustion Institute, 1739 (1990)), titanium nitride (I. Glassman, K. A. Davis, K, Brezinsky, Twenty-fourth Symposium (Int.) on Combustion, the Combustion Institute, 1877 (1992)), titanium diboride (D. P. Dufaux, R. L. Axelbaum, Combust. Flame 100, 350 (1995)), the tungsten carbide (G. Y. Zhao, V. V. S. Revankar, V. Hlavacek, J. Less Common Metals, 163 269 (1990)).
Since the early 90s the pace of research has been further intensified with a renewed interest in flame technology for manufacture of advanced materials with emphasis on nanosize particles. Matsoukas and Friedlander, J. Colloid. Interface Sci., 146, 495 (1991), observed that various ceramic particles made at about the same temperature in a premixed flat flame reactor exhibited distinctly different sizes attributing it to their different sintering rates and material properties.
Zachariah and Huzarewicz, J. Mater. Res., 6, 264 (1991), found that flame configuration may have a profound effect on the product powder properties. Specifically, they made submicron YBa.sub.2 Cu.sub.3 O.sub.7 particles by pyrolysis of the corresponding aqueous nitrate salts in an oxy-hydrogen diffusion flame reactor. They found that making these particles in an overventilated coflow diffusion flame resulted in superconducting powders while this was not the case when the particles were made in a premixed flame configuration at the same conditions! More recently, it was found that by merely altering the position of fuel and oxidant streams in methane-air diffusion flame reactors can change the average primary particle size of TiO.sub.2 powders made by TiCl.sub.4 oxidation by as much as a factor of 10.
The type of metal precursor did not affect the characteristics of SiO.sub.2 particles made in a counterflow or in a coflow diffusion flame reactor though it may affect the dynamics of particle growth (M. R. Zachariah and H. G. Semerjian, High Temperature Science, 28 113 (1990)). In contrast, during synthesis of GeO.sub.2 particles, the precursor can have a profound effect even on the characteristics of product particles. The process temperature has the most drastic effect on process and product characteristics (J. R. Bautista, R. M. Atkins, J. Aerosol Sci. 22 667 (1990)). The presence of additives or dopants can have a profound effect on the particle coagulation or sintering rate and subsequently on the characteristics of the product powder (Y. Xiong, S. E. Pratsinis, S. V. R. Mastrangeelo, J. Colloid Interface Sci., 153, 106 (1992)).
The existence of large temperature gradients in a flame can enact strong thermophoretic forces on the newly formed particles drastically altering their residence time at the decisive region where nucleation, growth, coagulation, sintering and oxidation occur affecting thus particle morphology, especially in counterflow laminar diffusion flames (A. Gomez and D. E. Rosner, Combust. Sci. Technol. 89, 335 (1993)).
Electrical charges can drastically affect the characteristics of aerosol made powders. Electric fields provide the unique opportunity for making powders with closely controlled specific surface area (D. R. Hardesty, F. J. Weinberg, Fourteenth Symposium (International) on Combustion, The Combustion Institute, 907 (1973)).
Patents which discuss the use of flame technology and nanoparticle formation include the following:
U.S. Pat. No. 5,494,701, Clough et al., issued Feb. 27, 1996, discloses processes for coating substrates, in particular substrates including shielded surfaces, with tin oxide-containing coatings. Such processes comprise contacting a substrate with a tin oxide precursor; preferably maintaining the precursor coated substrate at conditions to distribute and equilibrate the coating; oxidizing the precursor containing material to form a substrate containing tin oxide and contacting the substrate with at least one catalyst material at conditions effective to form a catalyst material containing coating on at least a portion of the substrate. Also disclosed are substrates coated with tin oxide-containing coatings for use in various catalyst applications.
U.S. Pat. No. 5,514,350, Kear et al., issued May 7, 1996, discloses an apparatus of forming non-agglomerated nanostructured ceramic (n-ceramic) powders from metallo-organic precursors combines rapid thermal decomposition of a precursor/carrier gas stream in a hot tubular reactor with rapid condensation of the product particles on a cold substrate under a reduced inert gas pressure of 1-50 mbar. A wide variety of metallo-organic precursors is available. The apparatus is particularly suitable for formation of n-SiCxNy powders from hexamethyl-disilizane or the formation of n-ZrOxCy powders from zirconium tertiary butoxide. The n-SiCxNy compounds can be further reacted to form SiC or Si3N4 whiskers, individually or in random-weave form, by heating in a hydrogen or ammonia atmosphere. The non-agglomerated n-ceramic powders form uniformly dense powder compacts by cold pressing which can be sintered to theoretical density at temperatures as low as 0.5 Tm.
U.S. Pat. No. 5,498,446, Axelbaum et al., issued Mar. 12, 1996, discloses a method and apparatus for reacting sodium vapor with gaseous chlorides in a flame to produce nanoscale particles of un-oxidized metals, composites and ceramics. The flame is operated under conditions which lead to condensation of a NaCl by-product onto the particles. The condensate encapsulates the particles and aids in controlling desired particle size and preventing undesirable agglomeration among the particles during synthesis. Following synthesis, oxidation of the particles is inhibited by the encapsulation and handling character of the products is greatly enhanced. Electron microscopy has revealed that synthesized products are composed of discrete nanoparticles in a NaCl matrix. The NaCl encapsulate has been effectively removed from the particles by both washing and known sublimation technique at 800.degree. C. under low pressure.
U.S. Pat. No. 5,368,825, Calcote et al., issued Nov. 29, 1994, discloses a novel process and apparatus for continuously producing very fine, ultrapure ceramic powders from ceramic precursor reactants in a self-sustaining reaction system in the form of a stabilized flame thereof to form ceramic particles and wherein the thus formed ceramic particles are collected in the absence of oxygen.
U.S. Pat. No. 4,994,107, Flagan et al., issued Feb. 19, 1991, discloses a method of producing submicron nonagglomerated particles in a single stage reactor includes introducing a reactant or mixture of reactants at one end while varying the temperature along the reactor to initiate reactions at a low rate. As homogeneously small numbers of seed particles generated in the initial section of the reactor progress through the reactor, the reaction is gradually accelerated through programmed increases in temperature along the length of the reactor to promote particle growth by chemical vapor deposition while minimizing agglomerate formation by maintaining a sufficiently low number concentration of particles in the reactor such that coagulation is inhibited within the residence time of particles in the reactor. The maximum temperature and minimum residence time is defined by a combination of temperature and residence time that is necessary to bring the reaction to completion. In one embodiment, electronic grade silane and high purity nitrogen are introduced into the reactor and temperatures of approximately 770.degree. K. to 1550.degree. K. are employed. In another embodiment silane and ammonia are employed at temperatures from 750.degree. K. to 1800.degree. K.
U.S. Pat. No. 4,891,339, Calcote et al., issued Jan. 2, 1990, discloses a novel process and apparatus for continuously producing very fine, ultrapure ceramic powders from ceramic precursor reactants in a self-sustaining reaction system in the form of a stabilized flame thereof to form ceramic particles and wherein the thus formed ceramic particles are collected in the absence of oxygen.
U.S. Pat. No. 4,604,118, Bocko et al., issued Aug. 5, 1986, discloses a vapor phase method for the synthesis of MgO--Al.sub.2 O.sub.3 --SiO.sub.2 products, including MgO--Al.sub.2 O.sub.3 --SiO.sub.2 glasses of optical quality, wherein SiCl4, aluminum halide, and organometallic magnesium vapors are oxidized by flame oxidation and the oxides collected and sintered to glass or ceramic products, is described. A added shield gas stream is provided during flame oxidation of the vapors to reduce or prevent MgCl.sub.2 by-product formation at the burner and in the product.
The paper "Vapor-Phase Processing of Powders:Plasma Synthesis and Aerosol Decomposition" which appeared in American Ceramic Society Bulletin Vol. 68 (1968) No. 5, pages 996-1000 discloses a glow discharge reactor with a pair of electrodes and a 13.56 MHz Rf power supply. It is described that the electrodes may be flat plate electrodes but a barrel reactor design is preferred. It is also disclosed that with glow discharges the atoms and molecules remain at room temperature.
EP-A-0124901 discloses a process for manufacturing fine powders of metal or ceramic in which a reaction gas is ionised by a high frequency induction coil and heated by laser beams. The heating can be carried out before or after ionisation.
It is an object of the invention to provide a relatively simple, high-production-rate method for effectively synthesizing high purity, nanoparticles of a uniform size in a continuous process. The invention disclosed and claimed herein achieves these advantages in a manner not disclosed or suggested by the prior art.